In the administration of whole blood to a patient it is desirable, if not necessary, to filter the blood as it is being administered. The debris which is to be removed by the filter will vary depending upon the length of time the blood has been stored. Over the years, many different filter media have been developed which are specifically useful in filtering debris from whole blood. Although the engineering of the filter media and the filter efficiency is extremely important there are a number of other problems involved with filtering blood during blood transfusions. First, in administering blood to the patient and in filtering this blood, the flow rate should be known and controlled. The filtration unit should be readily insertable into a blood reservoir, usually a blood bag, and the necessary administration sets for administering blood to the patient easily and readily connected to the filtration unit in a manner that does not disrupt or change the flow of the blood. The filtration unit should be constructed so not to collect or entrap micro-gas bubbles which might be administered inadvertently to the patient. The unit itself should have a low total volume so that blood is not held in the unit and all of the blood from a blood bag is administered to the patient. The filtration unit should be easily and readily primed and primed in such a manner as not to entrap gas bubbles. Along with all of these problems which should be overcome, the unit should have excellent filtering efficiency, do as little damage to the blood components as possible, and be usable for multiple blood transfusions to a single patient in order to be reasonably economical.